Sludge digesters typically include a storage tank for holding decomposing sludge, and a cover that contains, and floats on, an envelope of gas that is above, and generated by, the decomposing sludge. It is common to attach ballast to the cover to increase the weight of the cover, and the pressure of the gas within the digester. The pressure of the gas within the sludge digester is proportional to the effective weight of the ballasted cover. Examples of gas-holding storage tanks, such as sludge digesters, are found in U.S. Pat. Nos. 251,245; 1,989,589; 2,050,686; 2,050,915; 2,373,797; 3,187,897; 3,288,295; 3,535,236; 4,060,175; 4,378,437; 4,391,705; 4,648,968; 4,710,292; 5,092,482; 5,238,844; 5,384,033; 5,423,895; 6,247,278; 7,069,655; and U.S. Patent Application No. 2004/0232148, the disclosures of which are hereby incorporated by reference in their entirety for all purposes.
Most sludge digesters that include a ballasting system for the cover include horizontal support shelves that are attached and orthogonal to the lower end of the cover's side skirt, and that extend inwardly from the side skit toward the center of the storage tank. These support shelves are generally welded into place after the cover has been constructed and placed on top of the storage tank. In order to attach the ballasts to the support shelves, the ballasts must be lifted vertically from the base of the tank to a height that is greater than the support shelves, moved horizontally to a position above the support shelves, and then lowered onto the support shelves. The ballast is then secured to the support shelves where they are supported during operation of the digester.
This process presents a number of problems. First, because the support shelves are welded to the lower end of the side skirt after the cover is placed onto the storage tank, the support shelves and portions of the side skirt must be painted, while the cover is on the storage tank, so as to inhibit corrosion of the otherwise unpainted portions of the cover in the highly corrosive environment within an operating sludge digester. This is often very difficult, and costly, because some of the portions of the cover that need to be painted are not readily accessible, due to the tight fit between the side skirt and the outer wall of the storage tank. Some portions of the cover may not be painted effectively, thus increasing the risk of corrosion.
Second, because the ballasts must be lifted vertically, moved horizontally and lowered vertically in order to be secured to the support shelves, the ballasts must be moved in such a manner using either (a) a make-shift pulley system within the storage tank, which is difficult to use and construct due to the extreme weight of the ballasts, or (b) a crane or other lifting device that can only move the ballast onto the support shelves if the roof includes a elongated radial opening in its roof that permits the crane to lower and raise its cable or other lifting member vertically, and to move the ballast horizontally relative to the support shelf. If a lifting device is used through an elongate opening in the roof of the cover, then after the ballast is secured to the support shelf, and prior to operation of the digester, the opening must be sealed, such as by welding cover plates over the openings, to ensure that the cover is gas-tight.
Third, moving the ballast horizontally and vertically relative to the support shelves during installation of the ballasts creates a risk that the ballast will scrape paint off of the support shelves or the side skirt, which increases the risk that the support shelf will corrode and eventually fail. These problems are alleviated by the invention disclosed herein.